Testing units for micro electro-mechanical systems are known, essentially comprising:                a plurality of trays, each of which can be fed with a respective plurality of micro electro-mechanical systems;        a testing device for the micro electro-mechanical systems, which electrically communicates with the above-mentioned systems and verifies that the systems themselves are able to correctly respond to a plurality of external stimuli, for example correct transmission of the electrical signal, variations in temperature levels, humidity, noise, acceleration and vibration and magnetic fields; and        a collecting device, which collects the micro electro-mechanical systems from one of the trays and makes them available to the testing unit.        
More precisely, each tray comprises a plurality of chambers, for example a few hundred, within which the micro electro-mechanical systems to be tested are housed.
The trays enter the testing unit one after the other, stop, and the collecting device collects the micro electro-mechanical systems from the tray inside the testing unit.
The testing device comprises, in turn, a chamber provided with a plurality of seats, within which the micro electro-mechanical systems are housed and where their respective functions are tested by means of connection to electrical terminals.
Each micro electro-mechanical system further comprises, on its surface, a two-dimensional identification code, which contains numerous first pieces of information and can be associated with further second pieces of information contained in an external database, including the relative batch.
The Applicant has observed that the above-mentioned testing units, although they function correctly, could be improved, in particular as regards the rapidity and optimisation of the testing process.
More precisely, if the micro electro-mechanical systems are not correctly housed in the tray, there is the risk of the systems being positioned incorrectly within the respective seats of the testing device and consequently interrupting operation of the testing unit.
Furthermore, if the micro electro-mechanical systems are not correctly housed in the tray present inside the machine, there is the risk of interaction with the collecting device causing the micro electro-mechanical systems to come off the tray. This would cause non-collection of said systems or the generation of noise and vibration inside the testing unit.
Moreover, it is important to note that the micro electro-mechanical system must be correctly positioned in the seats of the testing device chamber, in order that said systems can be correctly connected to the electrical terminals.
If the micro electro-mechanical system is incorrectly positioned on the tray, it is nevertheless positioned by the collecting device inside the relative seat of the testing device and testing is performed, albeit ineffectively.
Lastly, if the micro electro-mechanical system is not present on the tray, due to an error, the collecting device detects this and collects another system, reducing the efficiency and rapidity of the testing operation.
U.S. Pat. No. 5,805,472 discloses a testing method comprising reading means for reading an identification code of test trays.
U.S. Pat. No. 5,865,319 discloses an automatic test handler.
The need is felt in the sector to optimise operation of the testing unit, in particular in terms of rapidity and optimisation of the testing operations of the micro electro-mechanical systems.